1. Technical Field
The present disclosure relates to an amplifier, in particular, to an amplifier circuit having a biasing block with output gain compensation, and an electronic apparatus using the amplifier circuit.
2. Description of Related Art
Amplifier circuits are widely used in the electronic apparatuses of various kinds, such as radio frequency (RF) communication apparatuses. One of the amplifier circuits has a cascode low noise amplifier (LNA). The cascode LNA has higher input-output isolation, low noise figure and high gain.
The cascode LNA generally has two N-type metal-oxide-semiconductor field-effect transistors (NMOSFETs), wherein the two NMOSFETs are cascaded with each other. One of the two NMOSFETs is operating as a common source amplifier, while the other one of the two NMOSFETs is operating as a common gate amplifier. The cascode LNA improves the input-output isolation (or the reverse transmission) as there is no direct coupling from the output to the input of the cascode LNA, thus eliminating Miller effect and contributing to a much higher isolation. Moreover, at least one source degenerator circuit can be coupled to the cascode LNA for the input and/or output impedance matching.
However, when a radio frequency signal input to the cascode LNA increases, a load line of the cascode LNA may enter a saturation region due to nonlinear characteristics of the cascode LNA. Thus, the output gain of the cascode LNA is compressed, the output power of the cascode LNA is saturated, and the harmonic distortion is caused. In addition, the cascode LNA should be coupled to a biasing circuit to receive biasing voltages, such that the biasing current may not be changed due to the temperature and/or system voltage variation.